1. Field of the Invention
The present invention is directed to a process for the production of powdered polyisocyanate addition products and to the powders produced therefrom.
2. Description of the Prior Art
The direct production of powdered polyisocyanate addition products is known as shown by U.S. Pat. Nos. 3,787,525, 3,817,886 and 3,917,741, and U.S. patent application, Ser. No. 07/336,978, which disclose thermoplastic powders based on polyisocyanate addition products, and U.S. Pat. Nos. 3,963,710 and 3,933,759, which disclose oligomers containing blocked isocyanate reactive groups. In general the powders based on polyisocyanate addition products prepared in accordance with these patents are prepared by a batch process wherein an insoluble reactant is first emulsified in an inert organic liquid and then subsequently reacted with a second soluble or insoluble reactant to form the solid reaction product.
One of the difficulties with the batch process is that the large reaction vessels required for commercial production require powerful stirring means to both emulsify the insoluble reactants into the inert organic liquid and to maintain a dispersion of the particles in the organic medium as the reaction to form the solid polyurethane takes place. In addition, the relatively slow rate of reaction of aliphatic isocyanates with hydroxyl compounds necessitates relatively long batch cycles. Further, the properties of the polyurethanes vary from batch to batch because of the complexities involved in the initial formation of the dispersed particles and the subsequent interfacial reaction of the individual components. Finally, it is often difficult to scale up a particular powder from a laboratory scale and still maintain the properties and appearance of the product as produced on the smaller scale.
Continuous processes for the production of isocyanate functional powders based on polyisocyanate addition products are known as shown by Kopp, et.al., in U.S. Pat. No. 4,680,367, issued July 14, 1987. However, this process is limited to isocyanate adducts containing urea groups.
Accordingly, it is an object of the present invention to provide a continuous process for the preparation of polyisocyanate addition products in the form of free-flowing, solid powders having properties and an appearance which are consistently reproducible. It is an additional object of the present invention to provide a continuous process which does not have the large power requirements of the known batch processes. It is also an object of the present invention to provide a continuous process for the production of powdered polyisocyanate addition products which is easy to scale up from laboratory scale to pilot plant scale and, ultimately, to commercial size production. It is a further object to provide a simple process wherein powder materials containing isocyanate functions and solely urethane (rather than urea) groups may be produced in a continuous process.
These objects may be achieved according to the present invention by using special low shear dynamic mixers as hereinafter described.